I debated this match-up a LONG time ago, I'm still giving it to the artists
Sasori
Sasori is the senior of two and relies on mostly poison based attacks with his puppets. He's the perfect partner when facing genjutsu like Itachi user since he has no nervous system to cast genjutsu on, and he fairs well against Kisame since he uses poison to kill instead of destructive chakra based attacks. Another benefit of Sasori's power that lets him also match Kisame's stamina is that Sasori can't tire out due to being in a puppet body and has 3 forms to get past. Like Deidara, he can also attack multiple opponents
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Deidara
Likewise, Deidara is another ninja with strange powers to counter in this fight. Deidara too has trained himself to counter genjutsu after his first fight with Itachi
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. This isn't the only thing that makes him well suited to fight Itachi and Kisame. Deidara's clay birds are also excellent for staying out of effective range from most of Itachi and Kisame's attacks. This is good since Deidara can support Sasori from above. The exploding clay is also a very effective against someone like Kisame. The clay is devastating not due to an energy based chakra attack, but the explosion itself is what is so dangerous. Another benefit Deidara has against this duo is that he can turn Kisame's greatest asset of turning the arena into a lake against them by turning it into a giant minefield
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Both of the artists have multiple attacks that allow them to destroy huge areas, thus taking out all clones, such as Iron Sand Unleash, C3, and C4. A C4 and Iron Sand unleash combo could potentially win the match there. ISU does two things: covers a wide area and limits mobility [
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][
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] for the other team. If Itachi and Kisame can't move quickly, they won't be able to escape the cloud of C4 bombs [
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].
If it comes to Itachi using Susanoo, then he'll sacrifice a good amount of his mobility and then he'll have whole other set of problems to face. Who said the C1 fish were only for Kisame? Susanoo is still vulnerable from below and Itachi gonna have to have to constantly watch himself from below and if his feet are injured, he won't be able to stand and water anymore and will be drowned by the weight of his own Susanoo, or from broken legs. Another weakness he gains to add is poison gas, which he won't be able to avoid as easily. Also, chances are he won't be able to use Susanoo for very long since he'll be pouring out chakra for that as well as gather chakra in his feat to stay above water.
Not to mention C3 would work just as well since the giant explosion would completely disperse the water with a giant explosion and would definitely kill Itachi since the mountain busting explosion would go off beneath him and Susanoo wouldn't protect him.